Modem systems operating at about 56 kbps were developed to take advantage of the fact that ISPs are connected to the PSTN through a digital line rather than a twisted pair of copper wires terminating at the central office. That is, referring now to FIG. 1, an ISP 120 generally comprises a server 102 coupled to a digital modem 144 which is connected to PSTN 108 through a digital line 134 and is capable of transmitting data at about 64 kbps. Data is transmitted to central office 110 over a digital line 132, and then over the analog local loop 134 to an analog modem 112 associated with a user system 114 (e.g., a PC or the like).
PSTN 108 comprises assorted networks and components used to provide, among other things, standard telephone service. PSTN 108 might include foreign exchange services (FX), local exchange carriers (LECs), inter-exchange carriers (IECs), digital loop carriers (DLCs) and the like. As the parameters of PSTN 108 and a line card coder/decoder (codec) provided within central office 110 are dictated by network specifications (e.g., the use of μ-law or A-law encoding), ISP modem 144 is configured to transmit digital data in such a way as to fully exploit its digital connection to the network. See, e.g., the ITU-T V.90 specification, hereby incorporated by reference.
Digital communication systems may employ a number of initialization, training, and adaptive learning protocols that are designed to equalize the channel distortions, optimize the data transmission speed, reduce transmission errors, and improve the quality of the received signal. For example, the current generation of pulse code modulation (PCM) modems, e.g., modem systems compliant with ITU-T Recommendation V.90, perform an initial training procedure to adaptively adjust the equalizer structure resident at client-side analog modem 112 (APCM).
V.90 modem systems perform an initial two-point training procedure during which one constellation signal point (based on a particular μ-law or A-law level) is transmitted as a sequence having positive and negative signs. The DPCM transmits the two-point training sequence to the APCM, and the APCM analyzes the received signal to determine the channel characteristics and to adjust its equalizers. After performing this two-point training, a digital impairment learning (DIL) procedure is performed.
FIG. 2 illustrates a typical V.90 modem downstream transmission channel 200 over which such a training sequence may be sent. The signal b(n) (202) may represent a sequence of digital symbols, e.g., 8-bit codewords, that are to be transmitted by a DPCM transmitter 204, where “n” represents the time index for the transmitted symbol.
A number of digital impairments 208, such as robbed bit signaling (RBS) and digital pads, may be present within the digital network channel associated with DPCM 204. A digital to analog conversion occurs at a PCM codec 212 to facilitate transmission to the end user over an analog loop as described above. Analog impairments 216, such as nonlinear and linear distortion, may be associated with the analog loop and/or any number of analog processing components. Furthermore, in practical applications, additive noise 218 may be introduced to the analog signal before the analog signal is received by the APCM receiver 220, which produces a series of estimated symbols b(n) (222).
Digital impairments significantly limit the performance of PCM modem receiver 220. Digital impairment includes, for example, digital pads and robbed-bit signaling (RBS). Since reliable operation of a PCM modem is predicated on PCM receiver 220 knowing which levels digital transmitter 204 is sending out, PCM receiver 220 must detect what type of digital impairment has been encountered on a particular telephone line, or estimate all available PCM levels. In typical V.90 systems, RBS-altered symbols are periodic in nature based on the symbol count; e.g., RBS may occur every six or twelve symbols. Furthermore, the effect of RBS is deterministic but unknown to the APCM, while digital pads cause a constant, deterministic, and level-dependent (nonlinear) effect.
Prior art modem systems may compensate for linear analog impairments, such as amplitude and phase distortions, with well known linear equalization techniques. Such techniques, however, may not adequately compensate for the presence of digital impairments, and may therefore alter the level associated with the predetermined training point. Consequently, the initial training procedure performed by known V.90 modem systems do not provide the most efficient and effective result.
Methods are therefore needed which overcome these and other limitations of the prior art.